The present invention relates to the manufacture of ophthalmic lenses and, in particular, to methods and apparatus for producing a curvature in a lens blank.
Traditionally, a curvature, such as a spherical curvature or a toroidal curvature having base and cross curve characteristics, is formed in a lens blank by cutting an initial curvature in the lens blank in a generating machine, and then finishing the lens in a finishing apparatus.
The generating machine utilizes a cupped grinding tool mounted on a swing arm to cut a concave curvature which closely approximates a prescription curvature. The finishing procedure utilizes a finishing tool comprised of a support or lap and flexible finishing pads mounted on a convexly curved surface of the lap. The finishing procedure usually comprises a multitude of steps, e.g., usually two fining steps and a polishing step, with different pads being used for each step.
The lap is chosen so that the finishing surface has a curvature (defined by the curvature of the lap surface plus the thickness of the pads) which substantially corresponds to the prescription curvature within a prescribed tolerance.
During the finishing process, the lens surface and finishing surface are mated, and the lens surface is brought to the curvature of the finishing surface. The length of the finishing period is a function of the amount of nonconformity existing between the curvatures of the finishing surface and the initially cut lens surface; the greater the non-conformance-the longer the finishing period. Usually, the amount of nonconformity is great enough to require that two fining steps be performed prior to the polishing step, i.e., a rough fining step and a fine fining step.
Recently, a lens cutting lathe has been developed which enables an initial curvature to be cut in a lens surface which possesses an extremely high degree of dimensional precision and smoothness. Such a lathe is disclosed for example in commonly assigned U.S. patent application Ser. No. 07/766,394. Notwithstanding the accuracy and smoothness with which the initial lens curvature may be cut, it is still necessary to bring that curvature to the curvature of the finishing surface. Hence, even if the lens surface were initially at precisely the prescription curvature, an extensive finishing period could still be required in cases where the curvature of the finishing surface does not correspond to the prescription curvature. This is often the case, since laps are generally manufactured by methods which are less accurate than the above-described lathe.
It would be desirable, therefore, to shorten the finishing period, especially by taking advantage of the recently developed lens cutting lathe.